Question: $\left(-4x - 9\right)\left(-10x + 5\right) = \ ?$
Explanation: $= -4x \cdot \left(-10x + 5\right) - 9 \cdot \left(-10x + 5\right)$ $= \left( -4x \cdot -10x \right) + \left( -4x \cdot 5 \right) + \left( -9 \cdot -10x \right) + \left( -9 \cdot 5 \right)$ $= 40x^2 + \left( -4x \cdot 5 \right) + \left( -9 \cdot -10x \right) + \left( -9 \cdot 5 \right)$ $= 40x^2 + \left( -20x + 90x \right) + \left( -9 \cdot 5 \right)$ $= 40x^2 + 70x + \left( -9 \cdot 5 \right)$ $= 40x^2 + 70x - 45$